Continuous coupling of tunable or broad band radiation into thin film waveguides

ABSTRACT

A technique is described for designing and constructing a thinfilm waveguide/coupler combination that will allow the continuous coupling of a beam of broad band or tunable radiation from a laser-like source into a thin-film waveguide that is deposited on a suitable substrate. This is achieved by selection of a combination of materials having proper refractive indices in a sandwich structure which comprises the waveguide resulting in high efficiency without resource to any mechanical realignment of the angle of incidence of the light beam relative to the waveguide as the frequency to be coupled varies.

United State;-

Midwinter 1 March 6, 1973 CONTINUOUS COUPLING OF TUNABLE OR BROAD BANDRADIATION INTO THIN FILM WAVEGUIDES Appl. No.: 125,094

[52] U.S. CI. ..350/96 WG [51] Int. CI. ..G02b 5/14 [58] Field of Search..350/96 WG [561 References Cited UNITED STATES PATENTS 3,489,481 1/1970Osterberg et al ..350/96 WG X H O R IZ.

OTH ER PU BLICATIONS Harris et ai., Beam Coupling to Films, Journal ofthe Optical Society of America, Vol. 60, No. 8, Aug. 1970, pp.1007-1016.

Primary Examiner-John K. Corbin Attorney-Arthur J. Plantamura andHerbert G. Burkard ABSTRACT A technique is described for designing andconstructing a thin-film waveguide/coupler combination that will allowthe continuous coupling of a beam of broad band or tunable radiationfrom a laser-like source into a thin-film waveguide that is deposited ona suitable substrate. This is achieved by selection of a combination ofmaterials having proper refractive indices in a sandwich structure whichcomprises the waveguide resulting in high efi'iciency without resourceto any mechanical realignment of the angle of incidence of the lightbeam relative to the waveguide as the frequency to be coupled varies.

3 Claims, 5 Drawing Figures CONTINUOUS COUPLING OF TUNABLE OR BROAD BANDRADIATION INTO THIN FILM WAVEGUIDES BACKGROUND OF THE INVENTION 60, page1,325, et seq. (1970); by J. E. Midwinter in the IEEE Journal of QuantumElectronics, Vol. Q46, page 583, et seq (1970); and by F. Zernike and J.E. Midwinter in the same journal, Vol. 60, page 577; and by J. H.Harris, R. Schubert and J. N. Polky in The Journal of the OpticalSociety of America, Vol. 60, page 1,008, et seq, (1970).

The aims of this technology are to permit the linkage of high-powergeneration capability for optical radiation contained in laser sourceswith the capabilities for processing of information using light which istrapped in a thin-film waveguide. The invention finds application, forexample, in the transmission of information, the impression ofinformation onto an existing optical carrier wave, amplification andfrequency shifting of radiation injected into the films, interactionsbetween streams of information impressed on optical beams of a dataprocessing nature, or simple measurements made with the opticalradiation in the guide of some characteristic of the optical waveguideor its surroundings. The invention finds particularly advantageousapplication in connection with this latter use.

In the disclosure which follows, the description is written in terms ofvisible radiation using the terms such as light and optical, although itwill be understood that the techniques provided by the invention areequally applicable to nonvisible radiation in the ultraviolet orinfrared spectrum by the choice of suitable materials that transmit atthose wavelengths. Additionally, the terms broad-band and tunable" areemployed herein to describe radiation whose frequency may span a largefrequency range so that it is not of significant consequence in designprocedures whether the range is covered by the simultaneous generationof many frequencies or by the generation of a single, but timevariable,'frequency.

In previous work, attention has been directed to the coupling of asingle frequency of radiation into a thinfilm waveguide, although thegeneration of other frequencies within the waveguide environment bynonlinear interaction and its subsequent coupling out of the waveguideenvironment into the surrounding space by a coupler system has also beenconsidered. The principle of operation of all the prior art devicesdescribed heretofore is essentially similar, i.e., the collimated beamof optical energy from a single transverse mode laser source that is tobe coupled into the thin film is allowed to be reflected by internalreflection from the inner surface of a high refractive-index surface.Some of the energy of that beam crosses the surface and appears outsideit in the evanescent wave. 1f the total reflecting surface is thenbrought near to the guide layer, the evanescent field begins topenetrate the guide and energy can flow from the impinging beam into theguide. The internal reflection is then only partial or frustrated. Bytenninating the high index surface at the edge of the impinging beam, itis possible to trap a large part of the beam energy in the thin filmguide. The termination mechanism of this frustrated total reflectionsurface comprises the element referred to as a prism which is placedcontiguous to the guide layer, i.e., the prism surface adjoining theguide layer is placed close to the guide but does not touch it.Moreover, the sur face of prism close to the guide layer may be eitherparallel to the guide or at some angle to it, and it is not essentialthat it be a plane layer but it may be curved in some constructions.However, the invention will be described in terms of a plane-surfacedprism placed with its frustrated reflecting surface parallel to theguide layer as illustrated in the drawing, since this is the simplestgeometry for purposes of analysis.

The conditions for maximizing the efficiency of trapping of energy inthe guide layer are important considerations; two conditions must befulfilledzone is that phase-matching must be achieved between theimpinging beam and the excited wave in the guide layer which is done byadjusting the angle of incidence of the impinging beam; the secondcondition involves careful adjustment of the size of the gap between thereflecting surface and the waveguide and the positioning of thetermination of the reflecting surface relative to the beamcross-section. Both affect the coupling efficiency and have beendiscussed in the prior art. 9

SUMMARY OF THE INVENTION The present invention is concerned with animprovement in the technique of light-wave coupling into thin films andmore particularly with an improved arrangement by means of which theprior art means of coupling a single frequency of radiation into thethin-film waveguide can be made so that the single frequency source canbe replaced by a tunable source such as a dye-laser or opticalparametric oscillator or by a broadband source of single transverse modecollimated radiation without loss of over-all coupling efficiency. Theobjectives of the invention are achieved without resource to anymechanical realignment of the source relative to the guide layer, whichwould in general have been necessary with a conventional coupler/guidesystem of the prior art.

ln accordance with the invention, a unit may be constructed by followingthe design and construction steps to be outlined hereinafter whichcomprises a rigid source/coupler waveguide system in which the frequencyof the source is allowed to cover a wide frequency band (typically threeto one in extent) and which affords efficient coupling of energy to bemaintained throughout this band once the system has been correctlyconstructed and set up. Such a capability was not contemplated by theprior art, which teaches rather that the device provides frequencyselectivity since phase-matching at a particular frequency depends uponthe use of a corresponding angle of incidence on the prism internalreflection surface, e.g. see U'.S. Pat. No. 3,584,230. ln accordancewith the invention, I have found, contrarily, that under certainconditions use may be made of this fact in the construction of certainnovel and useful light-wave coupling devices. The novel result isachieved through the careful design and construction of the variouscomponents that go to make up the coupler. The physical embodiment ofthe resulting coupler/waveguide system does not differ in most respectsfrom the system of the prior art; it differs essentially, however, inthe careful selection, preparation and deposition of material used inproducing the coupler/waveguide system. In general, the system of theinvention is directed to an arrangement in which the phase-matchinginvolved in the energy transfer process from the impinging beam to theguide wave or mode becomes essentially insensitive to the frequencychange. The technique is applicable to any of the various forms of priorart devices described in the abovenoted prior art disclosures. Forsimplicity, however, the description of the invention will be limited toa single geometry as shown in the figures of the appended drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the illustrative embodimentof the invention as seen by reference to FIG. I, the coupling angle canbe made essentially invariant with the frequency of the input radiationover a broad band of frequencies. This is to be achieved by theselection, in the first place, of materials of construction of theguide/coupler combination according to certain prerequisites and thesub- .sequent calculation of the correct guide thickness to produce therequired characteristic. The production of a guide of the requireddesign thickness is derived through the utilization of alreadyestablished techniques. However, the general form of the system to beconsidered will be as shown in the illustrative arrangement of FIG. 1. Asubstrate or cladding 11 is shown as providing support for the waveguidestructure. The substrate 11 has an index refraction n,. Superimposed onsubstrate 11 a thin layer 12 of higher index material (index n isdeposited. The layer 12 functions as the guide layer. Superimposed onlayer 12 is an upper cladding 13 which in general is relatively thickcompared to the guide layer 12. The layer 13 has an index of refractionn For the purposes of coupling radiation into the film, a prism 14 ofhigh index material will be used which is brought close to the guidinglayer 12 but not into contact with it. The index of refraction of theprism is n, which may be the same as the guiding layer n The uppercladding of the layer 13 with index n may be air or liquid or a solidmaterial deposited by any convenient means. The guide layer 12 isconveniently deposited such as by evaporation or sputtering; althoughother techniques may also be used. Also, while geometries other thanthat shown in FIG. 1 may be constructed in accordance with theinvention, the above is preferred because of the convenience ofmanufacture.

The light source 16 such as a laser is fixed at some angle 8 to theguide layer. Reference is made to the upper and lower claddings l3 and11, respectively without implying that the structure must necessarily beoriented in the specific manner shown.

BRIEF DESCRIPTION OF THE DRAWING A detailed understanding of theinvention will be obtained from the following description, together withthe drawing herein:

FIG. 1 is a layout drawing showing an illustrative embodiment of thegeneral form of the source/coupler prism-waveguide in accordance withthe invention.

FIG. 2 is a more detailed drawing of the waveguide layerand itssurroundings to define the terminology used in the description.

FIG. 3 is a plot of the angular deviation from constant coupling angleas the typical radiation source is tuned through the visible spectrumdemonstrating that the technique in accordance with the invention hasyielded a device that is essentially insensitive to frequency.

FIG. 4 is a plot of the far-field beam divergence of an impinging laserbeam as a function of wavelength across the visible spectrum if theutmost coupling efficiency is to be achieved referenced against thefar-field beam divergence derived from a simple confocal resonator.

FIG. 5 is a plot of the estimated coupling efficiency for a confocalresonator into the thin-film waveguide over the frequency/wavelengthrange studied, normalized to zero decibel at the band center.

In order to establish the necessary rules to provide the waveguidecoupling arrangement in accordance with the invention, it is firstnecessary to establish, by way of definition, the constraints that areapplied by the guidance process upon the propagation angle for a wave inthe guide. Referring now to FIG. 2, a wave in the guide 12 can beconsidered to propagate an an angle 4;, shown at 21, to the guidedirection by reflection at the top and bottom surfaces 22 and 23,respectively of the guide layer. Where the refractive index of the guidelayer 12 is n, and the refractive index of the coupling prism 14 is nthen the angles and 0 are related by Snell's Law as follows:

cos 0=(n /n cos (I) It is noted that the cosine appears in place of thesine because of the angle definitions adopted.

It is common practice to define the guided waves in the guidance layer12 and in the upper and lower claddings 13 and 11, respectively, in thefollowing form, for transverse electric mode in which the electricvector is purely transverse and along the direction of the Y axis andperpendicular to the X and Z axes. (FIG. 2) Then, the electric field Efor the three media is represented by the formula:

(III) where w is the frequency of the radiation in radions per secondand c is the velocity of light. Thus all the waves propagate in the xdirection as exp(ik,-x) and with transverse propagation in the zdirection described by the real and imaginary components respectively ofy, and [3, and B The relative electric field amplitudes A, B, B and Care determined by equating the electric and magnetic field components atthe boundaries, the absolute values are obtained from the power flowrelations (Poyntings vector) which, for the purposes of the presentinvention, are not critical. However, it is found that, in order to havea propagating mode of the guide, only certain discrete values of thetransverse propagation constants [3,, [3 may be tolerated. These areobtained from the solutions to the equation below.

hi "(l 1/71)+ 3/-/ (ix) where 2d is the mechanical thickness of theguide layer. Since this equation only has solutions for discrete valuesof k, corresponding to the allowed modes of propagation of the guidewhich are characterized by the integer values of m, only discrete valuesof d are allowed since (72 .r) (X) Detailed derivations for the aboveresults have been published heretofore, e.g., in the disclosures of P.K. Tien or J. E. Midwinter, supra.

The physical interpretation of the foregoing equation (IX) is readilyapparent and aids in understanding the operation of the broad-bandcoupler. The left-hand side of the equation represents half the phaseretardation suffered as transverse component of the guide wave wavetravels across the thickness 2d of the guiding layer. The two terms onthe right-hand side of the equation represent respectively half thephase change on total internal reflection of the guide wave at the lowerand upper boundaries of the guide layer. Thus, the equation requiresonly that the net round-trip phase delay of the wave be an integralmultiple of 2 1r, which is the appropriate condition for constructiveinterference ofthe wave.

The design conditions for broad-band operations are obtained by seekingsolutions for Eq. (IX) above such that for a fixed but selectivelydefined value of the guide half thickness, d, the ratio of /k does notvary significantly as the frequency of the wave is varied. One way,illustratively, of obtaining an approximate solution to the condition isto equate the differentials with respect to frequency of the leftandright-hand sides of Eq. (IX) after making a simple substitution asfollows:

9 equation from its differentials, we obtain the two results for the TEmode:

where 5 partial differential.

The following design procedures are employed in preparing the board-bandwaveguide coupler. The materials of construction are selected so thatthe refractive indices over the frequency band of interest and theirdispersion characteristics are known. It is necessary to ascertain thatat least one of the following conditions is satisfied (and preferablyboth) in order that Eq. (XIV) produces a physical solution:

E 21 XVI and/or "2 am m 50 Having done this, the numerical values forthe materials chosen for the substrate 11, guide 12 and upper cladding13 can be inserted into the equations (XIII) and (XIV) to obtain therelation between K and d. These equations can be solved simultaneously(most easily graphically) to obtain unique values of K and d which willcorrespond closely to those for broad-band operation, since in the firstorder they make the structure invariant with frequency. We have found inpractice that better values are obtained by solving Eq. (X- III) orequivalently Eq. (IX), to obtain exact values of the mode propagationangle d; at a variety of frequencies and for a variety of thicknesses inthe region around the value of d derived above. The better value ofd isthen chosen by inspection of the data to yield the most constant valuefor the band considered. This yields more accurate results, since thereare no approximations involved in ignoring higher differentials which inpractice contribute small but significant amounts.

The foregoing procedure derives the appropriate value of guide thicknessfor broad-band operation by assuming that the prism is constructed ofthe same material as the guide layer. In this case, r1 n, and 0 4a. Inthe event that a different material is chosen and one in which the ration.,/n does not vary by a large amount over the band of frequencies to bestudied, then the above design procedure can be repeated, initiallysolving for constant :1: and then solving equations (I), (XII) and(XIII) to yield values for (b and 0 with the different values of theguide half thickness d. The best choice of the half guide thickness forconstant 0 is obtainable by inspection but it will not be substantiallydifferent from that for the case of NF"; above.

Where the ratio m/n, varies by a large amount over the frequency bandunder consideration, the best choice quide thickness is likely also todeviate considerably from the value derived by the simultaneous solutionof equations (XIII) and (XIV).

The foregoing design procedure has been described for the transverseelectric mode of guide propagation. However, it may be applied directlyfor the transverse magnetic case also by making the following changes:

In the equations (II), (III) and (IV), By, the magnetic field, issubstituted for Ey, the electric field by way of field definitions.Equation (IX) becomes:

Thereupon, equations (XIII) and (XIV) become: for the transversemagnetic mode:

The design procedure followed hereinabove is used, except equations(XVIII) and (XIX) are substituted in place of equations (XIII) and(XIV). The conditions of equations (XV) and (XVI) must still besatisfied, but an additional condition is placed upon the materials tobe used, namely,

n (w 2 n )and/or n, Eng) xx) This condition follows from the need tomaintain a physical solution to equation (XIX).

These conditions being satisfied, the materials parameters aresubstituted in equations (XVIII) and (XIX) and the ensuing relationsbetween K and d are solved simultaneously for specific K and d values.Final optimization is carried out by more precise numerical solution ofequation (XVII) or (XVIII) using refrective index data over the wholefrequency band as before. By a repeat of the already describedprocedure, the case of a different prism material to guide material canalso be taken into account.

The physical operation of the broad-band coupler depends upon holdingconstant the round-trip phase delay for a wave propagating in the guideas the frequency of the wave is varied. Since 7 is almost directlyproportional to the frequency, we see that the left-hand side of thewaveguide phase or mode equation (IX) or (XVIII) is sharply dependentupon the frequency. This is balanced by making the right-hand side ofthe equations strongly dependent upon frequency by operating thewaveguide near to the breakdown of total internal reflection. Then, itis well known that the phase change on internal reflection becomes avery sharply varying function of either the angle of incidence or therelative refractive indices of the media. For a more detailedexplanation of this function, reference is made to Fundamentals ofOptics, F. A. Jenkins and H. W. White, Chap. 25, p. 515, 3rd Ed.,McGraw-Hill (New York) 1957. It is thus possible by the techniquesdescribed herein to balance the change in phase delay in traversing theguide by a change in phase shift on reflection brought about by thedispersion of the materials used as the frequency changes.

It is thus seen that a broad-band coupler may be designed andconstructed pursuant to guidelines provided hereinabove. It is importantto note that fairly accurate control of the variables must be adhered toat all stages if good performance is to be obtained. It is important notonly to use materials whose indices correspond to those used in thedesign but to recognize that the refractive indices of many materialsare different after evaporation or sputtering into a thin layer" whencompared to the bulk material. Where this occurs, it will be importantto use refractive-index data representative of the deposited materialrather than that of the bulk material. It will also be important tomaintain close control over the thickness of the evaporated orotherwise-deposited guide layer. In order to more fully illustrate thenature of the invention and the manner of producing the same, thefollowing example is presented wherein a particular case is examined todetermine the performance characteristics that are possible.

EXAMPLE Refractive-index data for two materials to representrespectively the prism and guide for one and the two claddings for theother are selected. The data used are taken from the Schott OpticalGlass catalog of Jan. l, 1971 published by Schott Optical Glass, Inc.,Duryea, Pa. 18642, which lists the refractive indices for a wide rangeof glasses over a useful wavelength range. The two glasses chosen arethe high index, high dispersion (SF6) and the low index, low dispersion(FKSO) for the guide and cladding respectively. No implication is to beintended that only glasses can be used in the construction; they arechosen purely because tabulated data are readily available. These dataare used to design a broad-band guide/coupler for TE, mode by thetechniques outlined above. The wavelength band from l.Ol microns to 0.48microns is examined thus straddling the visible region of the spectrum.

From the simultaneous solution of equations (XIII) and (XIV), a valuefor d of 0.04 units, where the unit of length was defined in terms of awavelength at 0.5 893 microns, i.e., d 2.36 10' cm. On solving equation(IX) alone for a variety of wavelengths and thicknesses in this region,it is found that for the band considered a better value for d was 0.0447units (d 2.634 10 cm.). Using this value for d, the values of the angle4) were calculated at each of the wavelengths considered over the bandand the angular error in the coupling angle, defined as the value of (bat the wavelength minus the value of d) at 0.5893 microns. The result ofthis evaluation is given in FIG. 3 which shows that to within 0.0l, thecoupling angle is constant. The assumption that n n, was made for thiscase so that 6 d).

The effect of deviations from the theoretical design in the refractiveindices of the materials used and also in the thickness of the guidelayer deposited was examined numerically. The result of this analysis isthat for the coupling angle 0 not to change by more than 0.001 radiansover a visible wavelength range, the thickness of the layer must becontrolled to about 5 percent and the indices of refraction of thematerials used must be accurate to about 0.005, both of which are withinthe range of control by present technology.

The effect of a widely varying frequency range on the power couplingefficiency of the thin-film guide/coupler was also examined and it hasbeen shown that for maximum coupling efficiency it is necessary to varythe far-field beam divergence of a single transverse mode laser beamentering the coupler prism according to the relation shown graphicallyin FIG. 4. Also, by using the beam taken from a fixed confocal resonatorand the variation in beam divergence associated with that, it has beenshown that without any further correction a variation of couplingefficiency of not more than 2 dB is expected over the visible band asshown in FIG. 5. A flatter power-coupling characteristic could beachieved by the use of beam-forming optics.

It will be apparent that various modifications may be made in thedetails presented by way of illustration without departing from thescope and spirit of the invention.

Having described my invention, what I claim as new and desire to secureby Letter Patent is:

l. A method for rendering the normally frequency dependent propagationangle for plane wavefronts in a thin-film waveguide substantiallyinsensitive to the frequency of the wave being propagated said waveguidecomprises sandwiching a layer of high refractive index (n betweenenvironments of two lower refractive indices (n,) and (u where materialsm, n n;, are chosen so that for the transverse electric mode ofpropagation, a first condition in which obtains and for the transversemagnetic mode of propagation the condition the center of the chosenoperating frequency band, and where K is equal to the cosine of thepropagation angle (1: for the guided wavefronts, the thickness of theguide layer of index n, 'being characterized such that for thepropagating mode the change in phase shift for the wave traversing theguide as the frequency changes is substantially balanced by the changein the phase shift on toiiil reflection at the boundaries of the guidinglayergzhe balancing of these relative phase shifts being judged over thewhole of the frequency band desired for operation.

2. A method for coupling, wherein the coupling angle is substantiallyindependent of frequency, a spatially coherent beam of broad-bandradiation into a thin film waveguide, which comprises introducing theradiation into said waveguide by means of a prism, said prism beingcontiguous to one surface of the waveguide, being constructed accordingto the method of claim 1 and of a material having a refractive index n.,the material and cut of said prism being such that the beam ofcollimated radiation enters the prism at normal incidence to arrive inthe vicinity of the guide at an angle 0 with respect to the guide layer,said angle 0 being related to angle at of the propagation of planewavefronts within the waveguide by the formula:

n, cos n, cos

material n, for said prism being chosen so that the ratio Fig/n remainssubstantially constant throughout the frequency band of operation.

3. The method of claim 2 wherein the thickness of the waveguide isadjusted so that the angle 0 in the equality n, cos (it n cos 0 remainssubstantially constant over the operating frequency band while the ration lm by selection of prism material is permitted to vary over the samefrequency band in which the guided wavefronts make an angle 4: to thewaveguide direction, said collimated beam entering said prism at normalincidence to arrive at an angle 0 with the waveguide direction.

t i 1 I t

1. A method for rendering the normally frequency dependent propagationangle for plane wavefronts in a thin-film waveguide substantiallyinsensitive to the frequency of the wave being propagated said waveguidecomprises sandwiching a layer of high refractive index (n2) betweenenvironments of two lower refractive indices (n1) and (n3) wherematerials n1, n2, n3 are chosen so that for the transverse electric modeof propagation, a first condition in which
 1. A method for rendering thenormally frequency dependent propagation angle for plane wavefronts in athin-film waveguide substantially insensitive to the frequency of thewave being propagated said waveguide comprises sandwiching a layer ofhigh refractive index (n2) between environments of two lower refractiveindices (n1) and (n3) where materials n1, n2, n3 are chosen so that forthe transverse electric mode of propagation, a first condition in which2. A method for coupling, wherein the coupling angle is substantiallyindependent of frequency, a spatially coherent beam of broad-bandradiation into a thin film waveguide, which comprises introducing theradiation into said waveguide by means of a prism, said prism beingcontiguous to one surface of the waveguide, being constructed accordingto the method of claim 1 and of a material having a refractive index n4,the material and cut of said prism being such that the beam ofcollimated radiation enters the prism at normal incidence to arrive inthe vicinity of the guide at an angle theta with respect to the guidelayer, said angle theta being related to angle phi of the propagation ofplane wavefronts within the waveguide by the formula: n2 cos phi n4 costheta material n4 for said prism being chosen so that the ratio n2/n4remains substantially constant throughout the frequency band ofoperation.